Two-tier architecture is the client-server model. In a two-tier environment you have a client requesting data directly from a database server across a network.
In a three-tier architecture there is a middle-tier placed between the client and the server, usually an applications server(s) which holds the business logic. In this case, the client normally sends the request to the middle tier and the middle tier forwards the request on to the database server and then we go back the other way again. The third tier is most often introduced in order to provide for scalability which you didn't get w/ the client server model.
Joe
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Joe Ramsey
Senior Database Administrator
dbaDirect, Inc.
(877)687-3227